1. Field of the Invention
This invention relates to zoom lenses having a range of 8-10 and an F-number of 1.4-2.0 or thereabout for use in video cameras or photographic cameras and, more particularly, to rear focus type zoom lenses of compact form while still getting a high zoom ratio and a large relative aperture.
2. Description of the Related Art
In zoom lenses for use in video cameras or photographic cameras, it has been known to make the focusing provision in a lens unit other than the first lens unit, when counted from the object side, and a wide variety of zoom lenses employing the so-called rear focus method have been proposed. The rear focus type zoom lenses, because of their focusing lens units being relatively small in size and light in weight, generally have the advantages that a weak driving power suffices for moving the focusing lens unit and that a rapid focus adjustment becomes possible.
Japanese Laid-Open Patent application No. Sho 62-24213, for example, proposes a rear focus type of zoom lens comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units, wherein the first and third lens units remain stationary, the second lens unit moves to vary the focal length, the fourth lens unit moves to compensate for the shift of an image plane resulting from the variation of the focal length, and a focusing is performed by moving the fourth lens unit.
By the way, in view of the embodiments of this reference, the second lens unit, which has the function of varying the focal length, is constructed from, in order from the object side, a negative single lens of meniscus form convex toward the object side and a cemented lens consisting of a negative lens of bi-concave form and a positive lens. This cemented lens chiefly corrects longitudinal chromatic aberration, spherical aberration and off-axial aberrations. Also, the distortion varies with zooming to as large an extent as 9-12%. Particularly at the wide-angle end, it reaches 5% or above in most cases.
In the above-described rear focus type of zoom lens, if a further reduction of the bulk and size and a valuable increase of the zoom ratio are aimed by strengthening the refractive power of the second lens unit, the burden of aberration correction on the cemented surface in the second lens unit becomes too large. It is, therefore, difficult to realize high performance throughout the entire zooming range.
The increase of the refractive power of the second lens unit calls for a smaller curvature of each of the surfaces of the lenses constituting the second lens unit, giving rise to additional problems on an aberration correction and an increase of the production cost.